14.23 Host

14.23 Host

The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URL given by the user or referring resource (generally an HTTP URL,
as described in section 3.2.2). The Host field value MUST represent
the network location of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the root "/"
URL of a server for multiple host names on a single IP address.

Host = "Host" ":" host [ ":" port ] ; Section 3.2.2

A "host" without any trailing port information implies the default
port for the service requested (e.g., "80" for an HTTP URL). For
example, a request on the origin server for
<http://www.w3.org/pub/WWW/> MUST include:

GET /pub/WWW/ HTTP/1.1
Host: www.w3.org

A client MUST include a Host header field in all HTTP/1.1 request
messages on the Internet (i.e., on any message corresponding to a
request for a URL which includes an Internet host address for the
service being requested). If the Host field is not already present,
an HTTP/1.1 proxy MUST add a Host field to the request message prior
to forwarding it on the Internet. All Internet-based HTTP/1.1 servers
MUST respond with a 400 status code to any HTTP/1.1 request message
which lacks a Host header field.